The invention relates to an uninterruptible oil supply system where oil is supplied from an oil storage tank during negative and zero gravity conditions with a minimum of additional components and existing equipment modification.
The invention described below relates to an oil supply system generally for an aircraft gas turbine engine that provides an uninterrupted supply of pressurised oil to the engine journal chambers for engine bearing chambers during any inverted flight, negative gravity, zero gravity or aerobatic manoeuvres of the aircraft.
In addition to military aircraft or acrobatic aircraft, which experience reversals of gravity during flight manoeuvres, a continuous supply of oil is required under all conditions in many commercial aircraft, especially those that incorporate journals rather than bearings. Roller bearings or thrust bearings generally have a greater capacity to self lubricate when oil supply is interrupted. The centrifugal force and rotational movement of components serves to create a vacuum and distribute oil during minor interruptions. On the other hand due to low cost and ease of manufacture, many critical rotating components are supported on journals consisting of oil impregnated bronze or brass sleeves, rather than roller bearings. Oil supply to journals is extremely critical oil is essential in avoiding excessive friction and heating. Lack of oil supplied to journals for less than one second will often completely destroy the journals and adjacent gearbox. For this reason, the present invention and many examples in the prior art have been proposed to guarantee an uninterrupted oil supply during all anticipated flight conditions.
Many different prior art systems have been designed to address the problem of oil starvation in the bearing or journal chambers when an aircraft experiences negative or zero gravity. Under normal flying conditions with the aircraft positioned in an upright attitude, oil within the oil tank falls under gravity towards a drain positioned at the bottom of the oil tank. A conventional oil supply system includes a pressure pump, which withdraws the oil from the tank and delivers it usually through a pressure regulating valve to the bearing or journal chamber. In the bearing or journal chamber oil spray nozzles or oil supply channels distribute oil to the moving parts to lubricate moving parts and absorb heat for cooling the parts. The centrifugal motion of the moving components scatters the oil to the outer periphery where it is gathered up in a scoop and withdrawn under vacuum by a scavenge pump. The scavenge pump conducts the return oil through an oil-air separator which removes air usually in a centrifugal manner. The regenerated oil is then returned to the oil tank for re-circulation.
Common air oil separators utilise centrifugal force to separate the oil and air due to the difference in density between the oil and air. Heavier oil is forced outwardly and the lighter air is vented from a central vortex area from the oil-air separator. An additional feature of most oil-air separators is the additional capacity to act as a pump, due to the energy imparted to the oil during the centrifugal movement of the oil.
Therefore in the prior art it is well recognised that the provision of an uninterrupted oil supply is essential to avoid the possibility of bearing failure, overheating and fire risk, or journal seizing.
Many prior art systems for providing uninterrupted oil supply are based on providing baffles or chambers within the oil tank itself to maintain a small reservoir of oil immediately adjacent the pump inlet during inverted flights. Understandably such systems have a limited reservoir capacity and therefore inverted flight or zero gravity can only be accommodated for a short period of time.
Further prior art systems relate to movement of the pump inlet and vents with rotating pendulums for example within the oil tank to ensure that the pump inlet and oil within the tank are in constant contact. These complex mechanical systems add substantially to the weight of the oil supply system. As experienced with any moving mechanical component, a pendulum within the oil tank involves the risk of malfunction and imposes the necessity of preventive maintenance. The above prior art systems have facilitated a limited degree of aerobatic manoeuvring but have not enabled aircraft to remain in orientations other than normal level flight for any significant length of time. Zero gravity or high gravity manoeuvres remain a challenge for these types of systems.
Further prior art systems provide multiple inlets within the oil tank itself, for example at the top, bottom and sides, all feeding towards a common valve. The valve itself utilises various gravity controlled plugs to open and close the alternative valve seats within the valve and ensure that oil is withdrawn from the corner of the tank where oil is forced and directed to the primary pressure pump. These systems are not completely reliable in that they depend on the physical movement of weighted components under the force of gravity of components valve mechanism itself. There is an inherent time delay in the movement of oil which may or may not be accurately matched by the time delay in the movement of the weighted valve plugs. Mechanical failure, jamming due to debris or clogging of the valve can prevent proper operation. Due to the extreme sensitivity of journals especially, these systems are not reliable enough for use in modern aircraft. The reliance on gravity motion to open and close the valve does not provide the degree of accuracy necessary for split second control of the oil supply system.
Examples of prior art gravity controlled ball valves are provided in U.S. Pat. No. 2,239,098 to Hunter, U.S. Pat. No. 2,312,495 to Soucek and U.S. Pat. No. 2,831,490 to Simcock. Examples of oil tank pendulum orientation systems are shown in U.S. Pat. No. 2,379,579 to Hunter, U.S. Pat. No. 2,800,975 to Carroll et al. and U.S. Pat. No. 3,011,504 to Klank Jr. U.S. Pat. No. 2,983,331 to Helsley Jr. shows an example of an oil tank with a cyclone oil-air separator and internal baffles to ensure adequate oil supply during inverted flight.
Taking a different approach however, U.S. Pat. No. 4,531,358 to Smith recognises that the breather or oil-air separator of the oil supply system can function under certain circumstances as an auxiliary oil pump. During inverted flight Smith provides that oil is drained from the bearing cavities towards the breather where air is removed. Absence of oil pressure in the primary oil delivery system provided by the pressure pump, results in passage of pressurised oil from the oil-air separator/breather past a check valve into the oil delivery conduits to the bearings.
A significant disadvantage however of the system described in U.S. Pat. No. 4,531,358 to Smith is that the oil trapped within the oil tank is not utilised during inverted flight or zero gravity. In effect the oil-air separator is used as an auxiliary pump to completely by-pass the oil tank. Oil scavenged from the bearing chambers is merely recirculated in a sub-section of the oil distribution circuit. Scavenged oil is directed through the breather, air is removed and the oil is directed back to the bearings by-passing the oil tank.
During relatively long periods of negative or zero gravity, the Smith oil system will reuse the oil circulating in the sub-section of the supply system many times and the oil reservoir trapped in the tank will remain unused. If such a system is used for a limited period of time the functioning of the oil system will be adequate provided there is no significant leakage or loss of oil. However, for extended periods of time the recirculated oil may overheat or accumulate a high concentration of debris thereby overtaxing the cooling system and filtering system of the oil circuit.
It is an object of the invention to provide an uninterrupted oil supply during inverted flight, negative gravity or zero gravity of substantially identical quality and quantity to the oil supply delivered during normal flight.
It is a further object of the invention to provide an uninterrupted oil supply system using existing equipment and with a minimal degree of additional circuitry valves and modification.
It is a further object of the invention to provide uninterrupted oil supply with a minimal additional weight, mechanical complexity, assembly cost and maintenance cost.
It is a further object of the invention to rationalise the multiple valves and control systems of prior art uninterrupted oil systems thereby resulting in lower manufacturing costs and maintenance requirements for the oil system.
Further objects of the invention will be apparent from review of the disclosure, drawings and description of the invention below.
The oil supply system according to the invention supplies oil under pressure to a bearing or journal enclosure during normal positive gravity conditions as well as negative gravity and zero gravity conditions.
A closed oil storage tank containing a volume of oil below a gas filled headspace with primary drain in a bottom portion normally supplies a pressure pump which passes a pressure control valve before entering an inlet of the enclosure.
An oil-air separator accepts return oil from the enclosure outlet via a scavenge pump, then separates and vents the air before returning the oil to the storage tank.
In negative gravity or zero gravity conditions oil is absent from the tank bottom and the primary pressure pump is starved of oil. The invention solves this problem with an auxiliary tank outlet disposed above the bottom drain (at the top and/or sides of the tank) and the oil-air separator serves as an auxiliary pump that also separates the air from oil before pumping the oil under pressure to the bearing or journal enclosure past a directional switch.
The directional control switch (1) connects the switch oil inlet to the switch oil return outlet and oil tank when oil pressure delivered by the pressure pump exceeds a predetermined minimum threshold pressure; and (2) connects the switch inlet to the oil supply outlet and enclosure when oil pressure delivered by the pressure pump is less than the threshold pressure.
The invention provides significant advantage over the prior art systems described above and in particular the system described in U.S. Pat. No. 4,531,358 to Smith. As mentioned above, Smith merely re-circulates oil recovered from the bearing chamber utilising the breather or oil-air separator as an auxiliary pump during zero and negative gravity conditions. The Smith system re-circulates the same oil and by-passes the large reservoir of oil retained in the oil tank during negative and zero gravity conditions. Any significant leakage or oil loss will rapidly deplete the oil supply in the Smith system. Whereas the Smith system re-circulates a small percentage of the oil continuously during negative and zero gravity conditions, the present invention has access the entire volume of oil thereby avoiding the risk of rapid leakage loss, or overloading of the oil filtering and heat exchanging systems.
A significant advantage of the present invention is the simplicity of the uninterrupted oil system. Existing equipment is utilised with minor modifications and minimal additional weight, mechanical complexity, additional conduits, valves and other components resulting in only a marginal increase in the manufacturing complexity, costs of manufacture and maintenance demands.
When compared to the system provided by U.S. Pat. No. 4,531,358 to Smith, the present invention rationalises the multiple valves and pressure control systems in a single valve with a single moving valve plug. Significant advantages result from use of a single valve such as avoidance of troubleshooting of multiple valves, minimising manufacturing and maintenance costs and enabling the valve, pressure pump and air-oil separator to be packaged within a compact unit that can be removed from service and replaced easily to avoid significant downtime for the aircraft during troubleshooting and maintenance.